1. Field of the Invention
The present invention relates to status monitoring systems such as burglar or fire alarm systems, and more particularly to status monitoring systems employing multiple sensors and special logic to reduce the probability of false alarms.
2. The Prior Art
Status monitoring systems using multiple sensors and logic circuitry to discriminate against false alarms are known to the art. Such logic circuitry generally accomplishes its function of avoiding false alarms by generating an alarm signal only if two or more of the sensors generate response signals within a predetermined interval of time. An example of such a system is found in U.S. Pat. No. 4,195,296, dated Mar. 25, 1980, issued to Galvin.
A problem with existing multiple-sensor status monitoring systems is the failure of such systems to give a trouble signal if a sensor malfunctions. A sensor malfunction may take the form either of a failure to respond to a stimulus or of a spurious response in the absence of a stimulus. The first kind of malfunction--failure to respond at all--can result in a failure to sound the alarm when the status being monitored changes. The second kind of malfunction--a spurious response--can result in a false alarm. Neither kind of sensor malfunction produces a trouble warning in existing multiple-sensor status monitoring systems, and hence there is no way to know that one or more sensors have malfunctioned until one or the other kind of system failure occurs.
Moreover, even if there is a system failure, if a sensor is malfunctioning intermittently there is no way to determine which of the various sensors is the cause of the trouble, and hence troubleshooting such a system failure is virtually impossible.
A partial solution to the problem of generating a trouble warning in the event of the first kind of sensor malfunction--failure to respond at all--is disclosed in U.S. Pat. No. 3,801,978, issued Apr. 2, 1974 to Gershberg. The Gershberg patent discloses an intrusion alarm system comprising the combination of a microwave motion sensor and an ultrasonic motion sensor. False alarms are avoided by activating an alarm only if both sensors simultaneously signal the presence of an intruder. The alarm is also activated if either sensor fails to function, but only a complete failure of either the microwave or the ultrasonic sensing signal causes alarm activation. So long as both sensors are radiating sensing signals, the failure of either sensor to respond to a proper stimulus will not be detected. A further limitation of the Gershberg system is that even in the event of a complete failure of one of the sensing signals, the Gershberg system does not identify the sensor that has failed.
Even the limited failure-detecting ability of the apparatus disclosed by Gershberg only works with an energy radiating sensor such as a microwave or ultrasonic motion detector. A passive sensor is not adaptable to being monitored by the Gershberg apparatus, and hence a failure of a passive sensor will not be detected by such apparatus.
A spurious response in the absence of a proper stimulus is easy to detect in a single-sensor status monitoring system because such a response activates the system's alarm. Since there is only one sensor, locating the fault is relatively simple once it has been determined that the alarm was a false alarm. However, a multiple-sensor system--even the Gershberg system--does not activate its alarm if it detects a response signal from only one sensor. A spurious response signal from any one sensor, regardless of whether the signal is continuous or intermittent, is simply ignored. Hence, since there is neither an alarm nor a trouble warning, the defective sensor will continue to malfunction and system performance will be degraded.
A partial solution to the problem of detecting a spurious response from one sensor is proposed in the multiple-sensor system disclosed in the Galvin patent. The Galvin system has logic circuitry to generate a first alarm signal if any one sensor is activated and to generate a second alarm signal only if at least two sensors are activated within a predetermined interval of time. Thus, if the first alarm, but not the second alarm, sounds, once it has been determined that the alarm was false, it will be apparent that one of the sensors has given a spurious response. However, in Galvin's apparatus there is no way to determine which sensor has caused the trouble.
It will be apparent from the foregoing that there is a need for a multiple-sensor status monitoring system having the ability to warn of a sensor malfunction either of the first kind or of the second kind and to identify the malfunctioning sensor. The present invention satisfies this need.